Skeletal System Overview

Introduction

The skeletal system is the structural framework of the body, providing support, protection, and facilitating movement. It also plays a crucial role in mineral storage and blood cell production.

• Skeleton: Framework that gives the body its shape and protects internal organs and soft tissues.

Components of the Skeletal System

• Bones: Rigid organs forming the main structure.

• Cartilage: Strong, flexible connective tissue at joints and other locations.

• Tendons: Connect muscle to bone.

• Ligaments: Connect bone to bone, stabilizing joints.

Functions of the Skeletal System

• Support: Provides structural support for the body.

• Protection: Shields vital organs (e.g., skull protects the brain).

• Movement: Acts as levers for muscles to produce movement.

• Mineral Storage: Reservoir for minerals, especially calcium and phosphate.

• Blood Cell Production: Houses bone marrow for hematopoiesis.

Cartilage

Types of Cartilage

• Hyaline Cartilage: Most common; provides support with flexibility. Found at articular surfaces of bones, nose, trachea.

• Fibrocartilage: Contains thick collagen fibers; resists compression. Found in intervertebral discs, pubic symphysis.

• Elastic Cartilage: Contains elastic fibers; maintains shape. Found in external ear, epiglottis.

Cartilage Growth

• Appositional Growth: Chondroblasts in the perichondrium add new cartilage to the surface.

• Interstitial Growth: Chondrocytes divide and secrete new matrix from within the cartilage.

Bone Tissue and Bone Cells

Bone Matrix

• Organic Component: Collagen fibers provide flexibility and tensile strength.

• Inorganic Component: Hydroxyapatite crystals (calcium phosphate) provide hardness.

Bone Cells

• Osteoblasts: Bone-forming cells; secrete bone matrix and initiate calcification.

• Osteocytes: Mature bone cells; maintain bone tissue and reside in lacunae.

• Osteoclasts: Large, multinucleated cells; resorb bone by breaking down matrix.

• Stem Cells (Osteoprogenitor Cells): Differentiate into osteoblasts.

Bone Remodeling

• Continuous process of bone resorption (by osteoclasts) and formation (by osteoblasts).

• Essential for bone growth, repair, and calcium homeostasis.

Bone Structure

Gross Anatomy of Long Bones

• Diaphysis: Shaft; composed of compact bone surrounding a medullary cavity.

• Epiphyses: Ends of the bone; mostly spongy bone covered by compact bone.

• Epiphyseal Plate: Growth plate; site of lengthwise bone growth in children.

• Periosteum: Dense connective tissue covering the outer surface of bone.

• Endosteum: Thin membrane lining the medullary cavity.

• Medullary Cavity: Central cavity containing bone marrow.

• Red Marrow: Site of hematopoiesis (blood cell formation).

• Yellow Marrow: Stores fat; found in adult long bones.

Microscopic Structure

• Osteon (Haversian System): Structural unit of compact bone; consists of concentric lamellae around a central canal.

• Lacunae: Small spaces housing osteocytes.

• Canaliculi: Tiny channels connecting lacunae for nutrient/waste exchange.

• Lamellae: Layers of bone matrix.

• Volkmann's Canals: Perforating canals connecting osteons transversely.

Bone Classification

Bone Shapes

• Long Bones: Longer than wide (e.g., femur, humerus).

• Short Bones: Nearly equal in length and width (e.g., carpals, tarsals).

• Flat Bones: Thin, flattened, often curved (e.g., skull, ribs, sternum).

• Irregular Bones: Complex shapes (e.g., vertebrae, pelvis).

• Sesamoid Bones: Develop within tendons (e.g., patella).

Bone Development and Growth

Ossification (Bone Formation)

• Intramembranous Ossification: Bone develops from fibrous membranes (e.g., flat bones of skull).

• Endochondral Ossification: Bone forms by replacing hyaline cartilage (e.g., long bones).

Growth Patterns

• Longitudinal Growth: Occurs at epiphyseal plates; responsible for increase in length.

• Appositional Growth: Increase in bone thickness.

Factors Affecting Bone Growth

• Genetics: Determines potential size and shape.

• Nutrition: Adequate intake of calcium, vitamin D, and protein is essential.

• Hormones: Growth hormone, thyroid hormone, sex hormones (estrogen, testosterone) regulate growth and development.

• Mechanical Stress: Weight-bearing exercise stimulates bone formation.

Bone Fractures

Types of Fractures

• Open (Compound) Fracture: Bone pierces the skin.

• Closed (Simple) Fracture: Bone does not pierce the skin.

• Incomplete Fracture: Bone is not completely broken.

• Comminuted Fracture: Bone shatters into several pieces.

• Impacted Fracture: One fragment is driven into another.

• Linear Fracture: Parallel to the bone's long axis.

• Transverse Fracture: Perpendicular to the bone's long axis.

• Spiral Fracture: Twisting force causes a spiral break.

• Oblique Fracture: Diagonal to the bone's long axis.

• Greenstick Fracture: Incomplete break, common in children.

• Stellate Fracture: Radiates from a central point.

Fracture Healing Process

1. Hematoma Formation: Blood clot forms at the fracture site.

2. Callus Formation: Soft callus of cartilage forms, then replaced by hard (bony) callus.

3. Bone Remodeling: Bone is reshaped to original form.

Calcium Homeostasis

Role of Bone in Calcium Regulation

• Bones act as a major storage site for calcium ions (Ca2+).

• Calcium is essential for muscle contraction, nerve function, and blood clotting.

Hormonal Regulation

• Parathyroid Hormone (PTH): Increases blood calcium by stimulating osteoclasts to resorb bone.

• Calcitonin: Lowers blood calcium by inhibiting osteoclast activity.

• Vitamin D (Calcitriol): Increases intestinal absorption of calcium.

Key Equation

Calcium homeostasis is maintained by the balance:

Axial Skeleton

Overview

The axial skeleton forms the central axis of the body, protecting the brain, spinal cord, and vital organs.

• Skull: Protects the brain and forms the structure of the face.

• Hyoid Bone: Supports the tongue.

• Vertebral Column: Protects the spinal cord.

• Rib Cage (Thoracic Cage): Protects the heart and lungs.

Skull Anatomy

• Cranial Bones: 8 bones forming the braincase.

• Facial Bones: 14 bones forming the face.

• Calvaria: Skullcap; forms the roof of the cranial cavity.

• Major Sutures: Coronal, sagittal, lambdoid, squamous.

Major Skull Features

• Foramen Magnum: Large opening for the spinal cord.

• External Auditory Canal: Passage for sound waves.

• Mastoid Process: Attachment for neck muscles.

• Temporal Lines: Attachment for temporalis muscle.

• Zygomatic Arch: Cheekbone structure.

• Mandible: Lower jaw; only movable skull bone.

Jaws and Teeth

• Maxilla: Upper jaw; forms part of the orbit and hard palate.

• Mandible: Lower jaw; articulates with the temporal bone at the temporomandibular joint.

Vertebral Column

Structure

• Cervical Vertebrae: 7 vertebrae in the neck.

• Thoracic Vertebrae: 12 vertebrae in the upper back.

• Lumbar Vertebrae: 5 vertebrae in the lower back.

• Sacrum: 5 fused vertebrae.

• Coccyx: 3-5 fused vertebrae (tailbone).

Intervertebral Discs

• Composed of fibrocartilage; act as shock absorbers between vertebrae.

Appendicular Skeleton

Overview

The appendicular skeleton includes the bones of the limbs and girdles that attach them to the axial skeleton.

• Pectoral Girdle: Clavicle and scapula; attach upper limbs to trunk.

• Upper Limbs: Humerus, radius, ulna, carpals, metacarpals, phalanges.

• Pelvic Girdle: Hip bones (ilium, ischium, pubis); attach lower limbs to trunk.

• Lower Limbs: Femur, patella, tibia, fibula, tarsals, metatarsals, phalanges.

Summary Table: Types of Bone Cells

Additional info:

• Some details, such as the precise number of bones in each region, were inferred based on standard anatomical knowledge.

• Hormonal regulation and fracture types were expanded for clarity and completeness.